A member of staff close to "C", Frank Stagg, said that he would never forget his bosses' delight when the Deputy Chief Censor said one day that one of his staff had found out that "semen would not react to iodine vapour".MI6 'used bodily fluids as invisible ink' (via Super Punch)
Stagg noted that "we thought we had solved a great problem".
However, the discovery also led to some further problems, with the agent who had identified the novel use having to be moved from his department after becoming the butt of jokes.
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.